Subterranean hydrocarbon wells have been traditionally thought of as being straight and vertical. However in many cases the wells possess some deviation from vertical, as this is almost impossible to avoid when drilling with pressure on the drill bit as it passes relative soft and hard formations. Slight doglegs are, in most instances, of little concern in the design and placement of the production equipment. With the advent of modern drilling techniques, wells are purposely deviated through large angles in order to penetrate the formation at a desired angle. Two of these methods of drilling are directionally drilled wells and horizontally drilled wells.
The directionally drilled well is typically started from a small area where a number of wells are clustered. Such wells were drilled vertically for a short distance, then `kicked off` and the `kick off angle` increased to achieve a desired run angle. The drilling continues substantially straight at the established run angle until it approaches the targeted zone or formation. The angle is usually decreased and the hole allowed to drop into the target zone. Typically the rate at which angle is built in these wells seldom greater than 7 degrees per one hundred feet.
The horizontal well is drilled for the purpose of penetrating the formation in a direction parallel to the formation rather than passing through it vertically. Such a well allows a much greater drain area. A horizontal well is accomplished by drilling vertically until the hole approaches the formation. The hole is then deviated until the hole runs parallel through the formation. Such wells are classified by the radius or "dogleg angle" that is used in intercepting the formation. The dogleg in a deviated casing is found by measuring the angle off of vertical at locations of interest along the run of the casing. These measured angles and the distance separating their location are used to calculate the "dog leg" angle and are expressed in degrees of change per one hundred feet of casing in that area. For example, the following radii are typical:
______________________________________ DISTANCE DOG LEG ANGLE ______________________________________ Short 30-45', 126-191 degrees per 100 feet Medium 300-500', 18.8-11.5 degrees per 100 feet Long 1200-1500', 4.8-3.8 degrees per 100 feet ______________________________________
This assumes that the path of the casing can be represented by an assembly of circular arcs of various radii. The actual deflection of the center of a length of casing can be calculated by the following approximation: EQU d=2.6(L/100)2.times.a.
Where:
d=deflection of section (inches) PA1 L=Length of section (ft) PA1 a=Dogleg Angle (deg/100 ft).
For those wells either deviated unintentionally or purposely directed either directionally or horizontally the drill pipe can usually pass easily through such deviations without damage, since a long string of drill pipe is quite flexible.
In many completed wells it is desirable to use a high flow capacity pumping system such as a submersible electric motor powered centrifugal pump (ESP). A typical ESP comprises different functional and separate parts; to-wit, a motor, an equalizer or sealing section, in some instances a gas separator, and a pump bolted together with flange joints. The motor rotatably drives a longitudinal and axial shaft which extends from the motor to the pump. Such systems are shown in U.S. Pat. No. 3,624,822 and Oil Dynamics Catalog ODI-1 (1986) "Total Pumping Systems". The purpose for such flange connected parts is for ease of shipping, field assembly, disassembly, and repair. However, to insert such an ESP system into a deviated well creates stress on the flanged joints. Because the flange type joints are weaker in bending than the rest of the unit, bending tends to concentrate in the weakest points with deflection and sometimes permanent deformation of the joint. If the yield strength of the material at the joint is exceeded, the unit will be permanently damaged. If the joint becomes damaged, it will put additional side load on the adjacent shaft bearing, drastically reducing the life of the equipment. To eliminate damage to the flange joint, its strength would have to equal the strength of the rest of the connected units. If such could be accomplished, the next weakest link, the bolts, would yield. Thus, the bolts would be stretched beyond their elastic limit.
There is yet a further consideration in inserting an ESP into and through the radius or arc of a deviated well. The cable which supplies the electrical power from the surface power supply must pass by the outside of the pump in order to reach the motor. It is attached to the outside of the pump and its connecting production tubing. As the ESP is pushed through the radius portion of the hole, it is important that the power cable is not pinched between the ESP and the well bore or casing. Heretofore to protect the cable in any type of well `cable guards` have been used. One such type uses a channel that covers the cable for its entire exposed length. Another method uses sets of `stand-offs` that are placed at the flange joints where the sections of the ESP ar bolted together. Such protectors are valid for most vertical wells but not for the severe bends of deviated wells.